Police said the children, aged eight to 11, fell ill after consuming a lunch of rice, soybean and pulses on Tuesday. The school, in the district of Chapra, provided free meals under the Mid-Day Meal Scheme, the world's largest school feeding programme involving 120 million children.

The school principal fled after the deaths became known and was dismissed, Bihar Education Minister P.K. Shahi told a news conference.

The exact cause of the deaths will be known only after the postmortem examinations and food sample tests, doctors and state officials said.

However, R.K. Singh, medical superintendent at the children's hospital in the state capital, Putna said,"We feel that some kind of insecticide was either accidentally or intentionally mixed in the food, but that will be clear through investigations."

"We prepared antidotes and treated the children for organic phosphorous poisoning."

"As soon as my boy returned from school, we rushed to the hospital with him. He was vomiting and he said his stomach was aching," said Raja Yadav, the father of one schoolboy.

Dozens of residents took to the streets in Chapra, pelting a police station with stones and setting ablaze buses and other vehicles, television channels showed.

"We are currently treating 27 children here," said R.K. Singh, medical superintendent at the children's hospital in the state capital, Patna. "Of them nine are in serious condition, 18 are stable."

Bihar Chief Minister Nitish Kumar has ordered an inquiry into the incident and has offered 200,000 rupees ($3,400) to the families of those who have died, the state's food minister said.

Bihar is one of the most impoverished states in India, according to government data.